First firm spectral classification of an early-B pre-main-sequence star: B275 in M17

The optical to near-infrared (300 – 2500 nm) spectrum of the candidate massive young stellar object (YSO) B275, embedded in the star-forming region M17, has been observed with X-shooter on the ESO Very Large Telescope. The spectrum includes both photospheric absorption lines and emission features (H I and Ca II triplet emission lines, first and second overtone CO bandhead emission), as well as an infrared excess indicating the presence of a (flaring) circumstellar disk. The strongest emission lines are double-peaked with a peak separation ranging between 70 and 105 km s−1, and they provide information on the physical structure of the disk. The underlying photospheric spectrum is classified as B6–B7, which is significantly cooler than a previous estimate based on modeling of the spectral energy distribution. This discrepancy is solved by allowing for a larger stellar radius (i.e. a bloated star) and thus positioning the star above